I just searched for 'Not replacing deleted config file /etc/dbconfig-common/bugzilla.conf' and found this link
, which seems to have a decent fix.
Apparently the chap in that thread fixed it by creating the conf file manually (/etc/dbconfig-common/bugzilla.conf in your case), and then doing an apt-get remove --purge bugzilla
followed by apt-get install bugzilla
Can you try that on your distro (using aptitude instead of apt-get, sorry I'm not aware if aptitude supports arguments like --purge) and see if it helps?